The invention relates firstly to a method for evaporating liquid, in which a hollow cylindrical drum, which is arranged in a housing and has a drum wall which comprise a liquid-absorbent material, is rotated about its cylinder axis which is arranged substantially horizontally, the drum being immersed in the liquid to be evaporated over part of its circumference, which liquid is situated in a trough which is arranged at the circumference of the drum, while a gas which takes up the evaporated liquid is conveyed in contact with the liquid-absorbent material.
A method of this nature is described in DE-A-2 362 619.
The said publication describes a method of the type indicated above which is carried out in a device which operates in the manner indicated above and in which the drum comprises a liquid-absorbent wick-like material which can be moistened with a liquid, for example water, in order that, when air is passed through the wick material from the inside of the drum outwards, water can evaporate.
A method of this type has the drawback that, when water is taken up in the wick material, the pores become filled with water and the resistance to the flow of air increases rapidly.
Although the above publication states that, after it has been immersed in the water, the wick material is allowed to drip dry, it is nevertheless necessary to reckon with a considerable resistance to the flow of air.
The object of the present invention is to provide a device of the type described above which does not present a drawback of this nature.
To this end, the method according to the invention is characterized in that the liquid-absorbent material delimits a large number of passages which extend across the thickness of the drum wall and the gas is conveyed through these passages.
In the present context, passages is understood as meaning openings with a cross section of considerable size with respect to the liquid-absorbent material included in the drum wall, as will be explained in more detail below.
Advantageously, the passages will extend substantially radially with respect to the axis of the drum; however, this is not imperative, since passages which run in a direction other than the radial direction will also be satisfactory, provided that they are sufficiently large.
With regard to the direction of gas flow inside the passages extending across the thickness of the drum wall, it is, of course, possible to select flow from the inside outwards and from the outside inwards; it is an attractive option for the gas to be supplied inside the drum and to flow through the passages to the space outside the drum.
Where the above text has referred to liquid and gas which takes up evaporated liquid, it will be understood that the method as described offers the possibility of evaporating a wide variety of liquids using an equally wide variety of gases.
However, many applications will involve the evaporation of water, while it is obvious to use air as the gas which takes up evaporated water.
Water and air are generally available in copious quantities, and these two media will be in widespread use in particular for industrial purposes; however, the invention is not limited to these two media. If, instead of water, another liquid is to be evaporated, it will at any rate be necessary to ensure that the liquid-absorbent material is able to absorb this other liquid and that the liquid-absorbent material is inert to the said liquid, i.e. is not chemically affected by this liquid.
In a first embodiment of the method according to the invention, it is used to cool process water which is fed into the trough and air from the environment is conveyed through the passages, and during rotation of the drum at least some of the process water which is absorbed by the water-absorbent material is evaporated, and in this way the temperature of the process water which remains therein and/or of the liquid-absorbent material is reduced, with the result that, during further rotation, the temperature of the process water in the trough is reduced as a result of the cooled remaining absorbed process water and/or the cooled liquid-absorbent material coming into contact with the process water in the trough.
Process water is generated in very considerable quantities in industry and considerable work is involved in suitably cooling such quantities of process water. In many cases, cooling towers are used, in which water drips downwards through a filling packing and air is passed through in countercurrent, in order to cool the process water through evaporation of water.
Cooling processes of this nature require large-scale plant which, furthermore, has the drawback of forming aerosols which are entrained in the gas flow and may reach the environment, which is undesirable for certain environmentally polluting types of process water.
The method according to the invention as described above makes process water cooling of this nature easy and, to do so, uses a plant of small dimensions. In the method according to the invention, the formation of aerosols is to a large extent prevented since there is no contact between a flow of air and water which is already finely dispersed.
The water is absorbed at the surface of the absorption material and will evaporate from this position; consequently, there is little likelihood of aerosols being formed. This embodiment of the method according to the invention will be explained in more detail in the examples.
In another embodiment of the method according to the invention, it is used to cool air, in which method air is withdrawn from the environment and is fed through the passages, and the trough contains evaporation water which is absorbed by the liquid-absorbent material and at least part of which is evaporated during the rotation of the drum, with the result that the temperature of the remaining evaporation water and/or of the liquid-absorbent material is reduced, with the result that the temperature of the air which comes into contact therewith is reduced.
Particularly in houses and buildings, it is important, particularly when the outside temperature is high, for the temperature of the air inside the buildings to be reduced to an acceptable level while, at the same time, providing sufficient ventilation. If the air is simply cooled and dehumidified, the inside climate becomes unpleasant, leading to what is known as sick building syndrome. This is because of the fact that, for economic reasons, the cooled air is reused, with the result that insufficient fresh air is supplied. The cooling efficiency of a good air conditioning installationxe2x80x94the COPxe2x80x94is 5. This means for each kW of electric power supply 5 kW of cooling energy is obtained.
Using the invention, outside air is cooled by the evaporation of water. The COP is in this case 75. The cooling energy is obtained by the evaporation of water. Since in this method the economic reason is virtually eliminated, it is possible to cool sufficient fresh air and to ensure that the inside climate remains cool and pleasant.
As will be explained in more detail below, by using the method according to the invention in which water is evaporated and therefore heat is extracted, it is possible to achieve very substantial cooling of air which is to be fed to a working environment while, at the same time, the humidity levels in the cooled air remain at an acceptable level. In the conventional air conditioning units of the type which have a closed evaporation/cooling system, it is often the case that not only is the air cooled, but also there is a considerable reduction in humidity, with the result that the condition of the air is uncomfortable for people working in this environment.
In another embodiment of the method according to the invention, it is used for desalination of water, in which method air is withdrawn from the environment and is fed through the passages, and the trough contains salt-containing water which is absorbed by the water-absorbent material and at least some of which is evaporated during the rotation of the drum about its cylinder axis, leaving behind salt and/or water which is further enriched with salt, while the water which has been taken up in the air in this way is recovered by condensation in a subsequent step, and heat and salt-containing water are added continuously in amounts corresponding to the amount of water which has been evaporated.
In this method, the absorbent material which is active in the method takes up salt-containing water, some of the water is evaporated, leaving behind salt or water with increased concentrations of salt, and then the water is recovered by condensation from the air flow which has been enriched with water vapour in this way.
In a highly attractive embodiment of the method for desalination of water outlined above, the condensation step which is present is carried out by using a separate cylindrical drum which comprises a water-absorbent material and which is rotated about its cylinder axis which is substantially horizontally oriented and part of the circumference of which is immersed in salt-free water which is situated in a separate trough and is at a temperature which is lower than the temperature of the water to be desalinated, the drum and the trough for the water to be desalinated and the drum and the trough for the salt-free water being accommodated in one chamber, while heat and water are extracted from the trough containing salt-free water continuously in amounts corresponding to the amount of water vapour which is condensed.
By selecting a lower temperature of the salt-free water than the temperature of the salt-containing water, it is possible to ensure that the water vapour pressure on the surface of the liquid-absorbent material on the salt-free water side of the installation is lower than on the salt-containing water side. Consequently, there is a driving force which causes water vapour to condense on the water-absorption material which is included by the drum rotating in a trough containing salt-free water.
In the attractive embodiment outlined above, it is ensured that the salt-free water is constantly at least 5xc2x0 C. lower than the temperature of the water to be desalinated.
By making a drum comprising water-absorbent material rotate through water which is to be desalinated and also making a drum comprising water-absorbent material rotate in a trough containing salt-free water, with both drums and troughs situated in the same chamber and with the water to be desalinated being at a higher temperature than the salt-free water, it is possible, under the conditions indicated, to obtain a simple, inexpensive desalination method which has a very beneficial effect on the cost price of salt-free water.
Where the above text has referred to desalination, a term which assumes the presence of salt, it should be understood that the method may, of course, also be used to obtain pure water from solutions which additionally or exclusively contain dissolved substances other than salt. The method can be used in situations in which the dissolved substance does not have any substantial volatility of its own.
In this context, it is also possible to mention the possibility of obtaining pure water from flows of sewage water; obviously, a process of this type must be surrounded by measures for disinfecting the contaminated water and/or the water obtained.
The word salt, incidentally, is understood as meaning not only NaCl but also the other dissolved minerals which are present in sea water.
The invention also relates to a device for evaporating liquid, at least comprising a housing in which there is a hollow cylindrical drum with a drum wall which comprises a liquid-absorbent material and which can rotate about its cylinder axis, which is substantially horizontally oriented, having a trough for liquid which is to be evaporated arranged at the circumference of the drum and means for conveying, in contact with the liquid-absorbent material, a gas which takes up the evaporated liquid, which is characterized in that the liquid-absorbent material comprises a corrugated liquid-absorbent material, an elongate strip of which, the longitudinal direction of which strip forms an angle with the corrugations of the corrugated liquid-absorbent material, is wound helically so as to form the hollow cylindrical drum, and the drum wall substantially consisting of liquid-absorbent material, the drum comprising passages which extend across the thickness of the drum wall, which are delimited by absorbent material and through which gas can be conveyed.
As indicated above, the passages will expediently extend substantially radially with respect to the axis of the drum: the angle between the longitudinal direction of the strip and the corrugations is then approximately 90xc2x0.
In an attractive embodiment of the device, the corrugated liquid-absorbent material is combined with a flat liquid-absorbent material to form a composite liquid-absorbent material. A material of this nature which is flat on one side, when wound helically to form a hollow cylindrical drum, will not exhibit any problems relating to the corrugations of the strip of material used which lie on top of one another becoming interlocked. Moreover, the presence of the flat liquid-absorbent material increases the active surface area of the drum still further, which is advantageous.
The liquid-absorbent material which is used in the invention may, of course, be any liquid-absorbent material which has a high liquid-absorption capacity at its surface and is expediently of board- or sheet-like nature; it is preferable to use a liquid-absorbent material which comprises a core of plastic material which is coated on two sides with an adhering plastic nonwoven material. By using a material of this nature, an advantageous uptake of liquid is obtained in the nonwoven coatings which adhere to the plastics material core, the liquid, such as for example water, being distributed in a uniform thin layer over the surface of the liquid-absorption material, and overall good wetting of the liquid-absorption material being obtained.
In a highly attractive embodiment, the core of the liquid-absorption material consists of polystyrene, and the plastic of the coating comprises polyester. A liquid-absorption material of this nature exhibits relatively little chemical interaction with a liquid, such as for example water; this means therefore that the water is substantially mechanically anchored. The material is highly advantageous since it has a relatively good chemical resistance, so that a drum fitted with material of this nature can be cleaned using moderate chemical agents.
The invention also relates to a cylindrical drum having a drum wall which comprises a liquid-absorbent material for use in a device as described above according to the invention, which is characterized in that the liquid-absorbent material comprises a corrugated liquid-absorbent material, an elongate strip of which, the longitudinal direction of which strip forms an angle with the corrugations of the corrugated absorbent material, is wound helically so as to form the cylindrical drum, the drum comprising passages which extend across the thickness of the drum wall and are delimited by absorbent material.
As described above, in an advantageous embodiment the corrugated liquid-absorbent material is combined with a flat liquid-absorbent material to form a composite material; the passages which are present in the drum will preferably extend substantially radially with respect to the axis of the cylindrical drum.